SIM lock issue in iPhone

Only the carrier the phone is locked to can unlock it. Contact the carrier to see if the phone qualifies to be unlocked.

Read this --> How to unlock your iPhone for use with a different carrier - Apple Support

In the less likely event that the phone is asking you for a SIM PIN or PUK code that you have forgotten, you will also need to contact the carrier. Guessing the wrong code too many times could make it so that you would need to ask the carrier not just for a new code, but a new SIM.

Use a SIM PIN for your iPhone or iPad - Apple Support